Gas turbine engines of the type used for industrial applications may employ combustor systems designed to minimize nitrogen oxide emissions. One such combustor system, disclosed in U.S. Pat. No. 5,481,866, entitled Single Stage Premixed Constant Fuel/Air Ratio Combustor, issued to Mowill on Jan. 9, 1996, is incorporated herein by reference to the extent necessary for a full understanding of such a combustor. The '866 patent discloses a combustor having an externally cooled non-perforated combustor liner that receives all combustion air from a venturi shaped premixer. Excess air that does not enter the combustor through the premixer is ducted so as to externally cool the combustor liner, and eventually re-enters the flowpath downstream of the combustion region through dilution ports. An air valve is used to directly control the amount of air supplied to the premixer so as to minimize nitrous oxide emissions at all power settings. The air valve has the effect of indirectly controlling the amount of air routed to the dilution ports.
A problem occurs when combustors of the type disclosed in the '866 patent are used in conjunction with an engine having a compressor with a relatively high compression ratio. At low engine power settings, the valve used to control air to the premixer is mostly closed forcing most of the compressed air through the dilution ports. Although engine power is reduced, the total volume of air being pumped by the compressor at low power or idle settings remains high, resulting in a substantial increase in dilution airflow at reduced power. However, the dilution ports are necessarily sized to provide adequate backflow margin at the lower flow, higher power settings. Thus at reduced power, higher dilution flow conditions, the dilution ports overly restrict the dilution airflow causing a larger than desired pressure drop across the combustor and a loss of engine efficiency.
One solution has been to provide a separate apparatus for varying the flow area of the dilution ports at different power settings in addition to a valve for controlling air to the premixer. A disadvantage is that such apparatus are typically very complex, adding significantly to the total cost of the combustor system.
Another solution is disclosed in the copending U.S. patent application Ser. No. 08/966,393, filed Nov. 7, 1997. This application discloses a combustor dilution bypass system that includes a valve and a low pressure drop combustor bypass duct. The valve simultaneously controls both the supply of air to the premixer, and the amount of air directed into a large bypass duct. Air entering the bypass duct is reintroduced into the gas flowpath as dilution air downstream of the primary combustion zone. At low power settings the valve directs most of the air to the bypass duct, in effect adding dilution flow to that provided through the fixed area dilution ports, whereby the pressure drop across the combustor may be controlled at an optimal level. This combustor dilution bypass system is illustrated in FIGS. 1-11 of this application.
Referring to FIGS. 1 and 3 there are two premix injectors 64 on each side of the combustor spaced about 180.degree. apart. Each premixer injector 64 injects tangentially a mixture of fuel and air into the combustion chamber 60. FIG. 12 shows the results from testing of this combustor and reveals two spikes in carbon monoxide generation as a function of angular distance around the combustion chamber 60. These spikes occurred just downstream of the venturis 70 and indicate areas of unburned fuel. This unburned fuel is believed to be caused by some of the mixture exiting the venturis flowing directly to the dilution zone 36 instead of being mixed and combusted in the combustion chamber. To see how this could happen, viewing FIG. 2, the fuel air mixture flowing from the premixer injectors 64 enters the combustion chamber tangentially and sprays outward upon entering the combustion chamber. Therefore some the mixture flows directly to the dilution zone 36 located at the exit of the combustor and does not mix in with the gas in the combustion chamber.
Accordingly, a need exists in a venturi to be used in these premixer injectors that would direct the flow of gas away from the combustor exit.